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Old 12-04-2016, 06:15 AM #1
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Lightweight Peat's Mucky Muck soil testing

Wanted to start a thread so we can learn from each other about soil tests for peaty, high organic matter, lightweight density soils, their interpretation and the differences between testing mineral soils vs. organic soils.

Organic soils or histosols are peat deposits. Usually forming in bogs or anywhere with poor drainage. The peats we use in our mixes are mined from these areas.

Peat bogs are also drained and with proper management, can produce high value crops. In the US, Michigan, Florida, North Caroilina and New York have substantial acerage of organic soil being put into agricultural production. These farms are also called muck farms. Whether in containers or in the ground, peaty, muck soils, organic soils, histosols, whatever you want to call them are different and have to be treated differently than mineral soils.

Let's find and understand the differences and the similarities.


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Old 12-04-2016, 08:49 AM #2
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First major difference between organic soils and mineral soil is bulk density.

Most mineral soils used in agriculture are from about 1 to 1.3g per cubic centimeter. The time and money saving calibrated soil scoops used by most labs assumes 1.18g per cc.

Organic soils have a much larger range of densities, .1 to .8 grams per cc. It Depends on the amount of minerals, clay or other things humans add to farm soil mixed in with the organic matter. As you can see, a much broader range. Soils over 20% organic matter are classified as organic soils.

Measuring bulk density for organic container soils is easy. Prepare a volume of field moist soil. A liter or a gallon should do. Good idea to take note of the weight at this point. Spread out and dry the soil with a fan on low. Be careful, when the peaty soil dries, the fan could blow some away. Dry the soil until its bone dry and loses no more weight. Take note of the final weight and volume. Convert to grams per cubic centimeter and you have the bulk density.

Bulk Density = mass of dried soil per volume of that soil when field ready. Usually expressed in grams per cubic centimeter.


Organic soils also shrink considerably more than mineral soils when dry. A scoop of dried organic soil represents a larger volume of field moist soil. This becomes important if the soil lab wants to test volumetricly vs by mass.
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File Type: pdf 144-Organic-Soils.pdf (209.0 KB, 32 views)
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Old 12-04-2016, 04:29 PM #3
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---Bulk Density continued.

Once organic soil is dry and shrunken down, the density goes up, once ground by the lab, it goes up further. Any attempt to test organic soils volumetrically must consider this.

https://www.nrcresearchpress.com/doi/...141/cjss81-054

Seems to be difficult for a lab to take a small dry sample and try to calculate or recreate the field moist volume that it once had. Much easier and more accurate ( if measured carefully and dried thoroughly) to determine in house.
This vastly reduced volume when dry vs the the much smaller volume reduction of mineral soil when dry is another important difference to consider when testing. It must be accounted for if testing with a volume of dry soil vs a mass of dry soil.*

*North Carolina labs that use Volumetric testing use an equation to estimate field moist volume of a dried weighed sample

Last edited by biggreg; 12-09-2016 at 08:26 AM.. Reason: NCDA update
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Old 12-04-2016, 05:17 PM #4
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Labs I've used and 99% of the labs in North America use a weight of soil for their extraction type tests. If your lab weighs in the soil,either with a balance or with a calibrated scoop, the report will show the amounts extracted in ppm, soil mass or mg per kilo.

If the lab uses a volumetric approach, and adds a known volume to the test, the report will be the amounts extracted in ppm, mass per volume or mg per dm3 or mg per liter.

In mineral soils, the ppm mass and the ppm volume would be equal if the bulk density is 1g per cm3.

In lighter weight organic soils this is not the case. In the same soil sample, Ppm soil mass will be a larger number than the ppm volume. It's important to know both.

https://www.naptprogram.org/methods

Last edited by biggreg; 12-09-2016 at 08:29 AM.. Reason: Accuracy
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Old 12-04-2016, 05:36 PM #5
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If a soil has 1000ppm soil mass of an element, that's 1000mg per kilo

If that same soil has a bulk density of:
1 g / cm3
Or
.5g / cm3
Or
.2g / cm3

Then the ppm volume ( mg per liter) would be:
1000 mg per L
Or
500mg per L
Or
200 mg per L

In mineral soil, the difference Between soil mass ppm and mass per volume ppm is so close, its usually not considered.

For our organic, light weight, peaty, mucky mucks, this is very important as plants grow in a volume of soil, not a weight of soil.
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Old 12-04-2016, 05:57 PM #6
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For any extraction test done by the mass of soil, an accurate measurement of the required mass is important. If the lab mistakenly uses their usual mineral soil calibrated weighing scoop to measure the mass of some organic, lightweight soil, it will not be even close to accurate. Standard mineral soil weighing scoops are a time and money saver for the mineral soil in which they are calibrated. The scoop assumes a 1.18 g per cc density. This is where labs not experienced with organic soils go way wrong and enter grossly inflated mass measurements into the soil mass conversion equations. These equations assume the test added the required amount of soil. One cannot scoop to weigh organic soil unless one calibrated a scoop's volume to approximate the required mass needed for a proper test. Due to the large variability of organic soil density, a calibrated organic soil scoop is not practical in most cases.
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Old 12-04-2016, 06:35 PM #7
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if one is experienced at looking at mineral soil tests, the ppm ( soil mass) of an organic soil test ( in which the soil was weighed properly) may appear shocking.

If one attempts to evaluate the lighter weight soil by ppm without considering the bulk density and the ppm volume, one could draw wrong conclusions. For organic soils, we have to think volumetrically to compare nutrient levels with nutrient levels in mineral soils. 6000ppm of Ca may be way too much in mineral soil but wouldn't be enough in a high CEC, low density organic soil. If that 6000ppm ( soil mass)Ca organic soil had a bulk density of .2 g/cm3, it would have the same mass of Ca per liter ( 1200mg) as a mineral soil that was 1g/cm3 and only 1200ppm( soil mass) It's all proportions and relative. Big change from looking at mineral soil tests.

The practice of converting ppm to pounds per acre assumes 2 million pounds per acre slice must be adjusted of organic soils. An organic soil may only weigh 400,000 lbs per acre slice.

Bulk density is key to evaluating pounds per acre and for converting soil mass ppm to ppm volume in organic soils.
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Old 12-04-2016, 07:05 PM #8
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One must also be mindful of this proportional relationship with bulk density when looking at CEC.

The CEC of a good mineral soil may be 20 meq per 100g

The CEC of a mucky muck soil may be 100 meq per 100g

To know which soil can hold more cations on its exchange sites, we again have to look relative to its bulk density.

The mineral soil may be 1g / cm3 and the organic may be .2g / cm3

To compare we must look at CEC volumetrically. Meq per liter or move the decimal, meq/100cm3

That mineral soil at 20 meq per 100g and 1g/cm3 is 200meq per L or 20meq/100cm3
That organic soil at 100meq per 100g and .2g/cm3 is 200meq per L or 20meq/100cm3

* please note that if the soil mass of an organic soil was measured with a mineral soil calibrated scoop and the actual mass is less than assumed by the test calculations, , the CEC of the report will be low by a large margin.

In this case, each soil has the same number of exchange sites per volume.
Attached Files
File Type: pdf CEC and CN ratio.PDF (69.7 KB, 12 views)

Last edited by biggreg; 12-09-2016 at 04:43 AM.. Reason: More scoop bashing
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Old 12-04-2016, 07:54 PM #9
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Labs may use mineral soil conventions in their interpretations of the ppm results of an extraction of organic soil.

Some labs may cap Calcium at 15 meq. This practice is practical for a typical mineral soil farm. Usually when the lab sees more than 3000ppm Ca in mineral soils, the field probably has been recently limed. Any recent liming will skew any extraction. Sufficent time must pass after applying amendments before testing.

This practice is not applicable to organic soils. An organic soil could have 15 meq of Calcium and still be quite acidic and without free Ca carbonates.

When using lab results for organic soils, we must be aware the lab may not have experience with these soils. We must look at their methods carefully

Last edited by biggreg; 12-09-2016 at 10:28 AM.. Reason: Accuracy
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Old 12-04-2016, 08:38 PM #10
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Many Labs use a 1:1 volume soil:distilled water to check ph.

The bulk density of organic soils affects this as well. It's common to use 20ml water to 20ml soil. Again with mineral soil, this is also about 1:1 mass. With organic soils, the same volume:volume ph test may be 1:5 soil to water. The more dilute soil solution will have a different ph reading.

I noticed labs reporting back my ph much higher than my at home readings. I can accept +-.2 but not .6 or .7 higher.
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